For it's time the Mig-21 was pretty advanced, but the F-4 Phantom was made to counter the Mig-21. However Soviets have a history of selling watered down weapons to their customers. I believe most export fighters from the USSR lacked advanced electronics and only had bare minimum equipment.
As for the NATO bombing campaign in the north, it was very effective. If your enemy is asking for a cease fire, then you are doing something right. The Gulf War and Libyan war also had similar results where all you needed was large fleets of bombs and fighters to counter a technologically weaker conventional army. The Yugoslav wars had different results, NATO peace keepers found themselves having a hard time trying to bomb the small moving bands of Serb troops as well as the always moving SAMs. Against an equal force like the Russian Federation however, the US Airforce would probably get shredded up by their advanced air defenses and air force.
The North Vietnamese also probably faired slightly better than the use in conventional ground warfare. They had light amphibious PT-76 tanks and they were able to attack places where a tank assault wasn't expected. Also the Army and USMC were in for rude awakening when they found that the M72 LAW was ineffective against a lightly armored tank such as the PT-76. However in a tank on tank battle the M48 Patton could kill a PT-76. It was Equal to the T-54s operating in the north, but good luck getting those through the marshes, otherwise you are relying on really long roads to get you were you need to go.
